An engine may typically operate near stoichiometric conditions to improve efficiency of a catalyst that processes engine output emissions. If the engine is operated at higher engine speeds and loads with a stoichiometric air-fuel ratio, exhaust temperatures may rise above desired temperatures for exhaust system components. For example, exhaust temperatures may increase above desired catalyst temperatures or exhaust turbine temperatures. Therefore, it may be desirable to operate the engine in a way that limits exhaust temperatures to less than a threshold temperature. One way to reduce engine exhaust temperatures is to operate the engine with a rich air-fuel mixture. The rich air-fuel mixture reduces combustion temperature and extracts thermal energy from the engine, thereby cooling the engine and exhaust. Nevertheless, operating the engine with a rich air-fuel mixture may degrade vehicle emissions since three-way catalysts provide highest exhaust gas conversion efficiency when supplied exhaust gases produced by engine cylinders operating with near stoichiometric air-fuel ratios. Consequently, it may be desirable to operate an engine in a way that provides low emissions while operating below a threshold exhaust temperature at higher engine speeds and loads.
The inventors herein have recognized the above-mentioned disadvantages and have developed an engine operating method, comprising: operating a Miller cycle engine with fuel injected in an exhaust system of the Miller cycle engine at a location upstream of a turbine coupled to a crankshaft of the Miller cycle engine; and passing exhaust gases from the Miller cycle engine through the turbine.
By injecting fuel upstream of a turbine to provide a rich exhaust gas mixture, turbine vane temperatures may be reduced so that the Miller cycle engine may be operated at lean best torque rather than rich best torque during high engine speed and load conditions. Consequently, the engine may use less fuel at higher engine speeds and loads while exhaust system components temperatures are constrained. Further, in some examples, the rich exhaust gas mixture may be ignited so that the turbine delivers torque to a vehicle powertrain. Further still, the rich exhaust gas mixture may be combined with air at a location upstream of a catalyst to provide a stoichiometric exhaust gas mixture to the catalyst to provide high catalyst efficiency. In this way, engine exhaust temperatures may be maintained lower than a threshold temperature to reduce the possibility of exhaust system component degradation without reducing catalyst efficiency.
The present description may provide several advantages. For example, the approach may reduce the possibility of exhaust system component degradation. Further, the approach may reduce vehicle exhaust emissions at higher engine speeds and loads. Additionally, the approach may increase vehicle power during higher driver demand conditions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.